Back flow preventers are required equipment in all potable water supply lines where there is any possibility of reverse flow back into the supply line due to changing pressures in the supply and outlet lines.
Back flow preventers now in use generally include two check valves arranged in series so that water under sufficient pressure on the supply side will cause the valves to open and allow water to flow freely to the outlet pipes. It is also customary practice to include in the passage between the check valves, a relief valve which will automatically open should there be any flow in the reverse direction due to a drop in the supply line pressure and failure of one or both check valves to hold against higher outlet pressure. With the relief valve open, any contaminated or polluted water passing the downstream check valve in the reverse direction will be drained to atmosphere and thereby be prevented from entering the supply line.
The following United States patents are illustrative of the present state of the art: Carlton U.S. Pat. No. 2,389,413; Snyder U.S. Pat. No. 2,503,424; Curtiss U.S. Pat. No. 3,448,763; Tine U.S. Pat. Nos. 3,636,968; 3,747,621.